Recently, there is a growing need for cold forging that can reduce or abbreviate machining such as cutting, for improvement in productivity. As compared to hot forging, cold forging has a problem that deformation resistance is high, and deformability (ductility) is poor, thus there are problems that mold crack and steel crack are likely to be caused.
Therefore, the steel material to be subjected to cold forging is generally subjected to spheroidizing annealing aiming at reducing deformation resistance and improving deformability. Patent Literature 1 discloses a wire rod or steel bar having excellent cold workability, that is softened by specifying the ferrite fraction to have low deformation resistance even as hot rolled.
In addition, it is known that deformability after spheroidizing annealing is strongly affected by a structure before spheroidizing annealing, i.e., pre-structure. For example, Patent Literature 2 discloses a method for improving deformability by using a pre-structure having a pro-eutectoid ferrite fraction of 5 to 30% by area, with the balance comprising a structure mainly consisting of bainite, and in which, also, the average value of the lath interval of cementite in the bainite is set to 0.3 μm or more. Also, Patent Literature 3 discloses “steel wire rod or bar steel for case hardening having excellent cold forgeability after spheroidizing” in which refinement of carbide is possible when performing spheroidizing annealing and having high deformability by having a mixed structure comprising ferrite, bainite and pearlite and specifying the area fraction of the bainite to 30% or more. In addition, Patent Literature 4 discloses an invention in consideration of preventing crack during cold working for the structure after spheroidizing annealing by specifying the ferrite fraction of the surface layer structure to 10% or less.